In present technology wherein ring laser gyroscopes are employed in guidance systems, those of ordinary skill in the art recognize that, if there is any scattering within the lasing cavity of a ring laser gyroscope so that a portion of one of the counter-rotating laser beams couples into the other beam, there is a tendency for the phases of the two beams to pull towards each other. This pulling effect is most apparent when the frequencies and phases of the two laser beams travelling in opposite directions within the same cavity are close to each other, such as when the input rate approaches zero. This pulling of the frequencies and the phases of the two beams towards each other, frequently referred to as lock-in, results in an error in the output of the gyroscope.
One means available in the prior art to reduce this pulling between the beams is to modulate the ring laser gyroscope about its input axis in a periodic fashion. This modulation is frequently referred to as dither, as disclosed in J. E. Killpatrick U.S. Pat. No. 3,373,650 issued Mar. 19, 1968, and assigned to the same assignee as that of the present invention. In the patent there is disclosed that the dither motion reduces the lock-in effect the majority of the time. A further development in the dither technique is disclosed in U.S. Pat. No. 3,467,472, also issued to J. E. Killpatrick and assigned to the same assignee as that of the present invention, which teaches that the error due to lock-in can be further reduced by randomizing the oscillation or dithering of the beams so that the small errors at the extremities of the oscillation are no longer cumulative. However, those of ordinary skill in the art will recognize that, even with the significant benefits derived from the utilization of the teachings of U.S. Pat. Nos. 3,373,650 and 3,467,472, there is still a small error in the angular output of the ring laser gyroscope which manifests itself as a drift with the characteristics of a statistical random walk. This angular random walk error, of necessity, affects the accuracy of the measurements being conducted.
T. J. Podgorski U.S. Pat. No. 4,152,071, issued May 1, 1979 and assigned to the same assignee as that of the present invention, discloses that random walk can be reduced by shifting the position of the mirrors in the gyroscope so as to change the position of the path that the laser beams traverse within the lasing cavity of the ring laser gyroscope. By shifting the position of some of the mirrors, for example, one mirror can be shifted inwardly while another can be shifted outwardly, it is possible to reduce the random drift or angular random walk of the gyroscope while maintaining the same laser path length. This is accomplished because the new path that the laser beams traverse moves the beams away from any sources of scatter within the lasing cavity.
Though the technique disclosed in U.S. Pat. No. 4,152,071, generally serves to reduce the random drift of a ring laser gyroscope, those of ordinary skill in the art will recognize that the technique has certain limitations. For example, the discriminant signal, referred to as the single beam signal or the AC component of the laser power signal, which is used to determine whether the gyroscope is operating at a minimum in angular random walk, as discussed in U.S. Pat. No. 4,152,071, is only modulated in a readily measurable manner if the ring laser gyroscope experiences a zero input rate in a periodic manner. For example, if the input rate applied to the ring laser gyroscope does not exceed the peak dither rate, then the ring laser gyroscope will experience such zero input rate in a periodic manner. However, if the ring laser gyroscope does not experience zero input rate for a sufficiently long time, then the techniques discussed will not work.
It will also be apparent to those of ordinary skill in the art that, since real gyroscopes are not ideal, all gyroscopes indicate a non-zero output for zero input rate. This non-zero output is referred to as bias. This bias is not a major problem if it is constant for all conditions, since it can be compensated for. However, it has been discovered that the bias in the output of a ring laser gyroscope may change with changes in the above-mentioned single beam signal. This indicates that there is a different gyroscope bias associated with each potential path that the laser beams may traverse within the lasing cavity, since each of these paths result in a different scatter of one of the beams into the other giving a different resulting single beam signal. It has also been discovered that this single beam signal and the angular random walk of a ring laser gyroscope change with changes in the gyroscope temperature. Consequently, controlling the position of the path that the laser beams traverse within the lasing cavity for a constant angular random walk, with the minimum being an example of a constant random walk, also minimizes the changes in the gyroscope bias. As an additional consequence, controlling for a constant angular random walk over temperature also results in a minimum variation in the gyroscope bias. Further, certain specific paths for certain temperatures, when selected will result in minimized bias with a constant, though not necessarily minimum angular random walk.
Accordingly, in accordance with the invention, the problems of minimizing angular random walk and compensating for or minimizing bias in the referenced techniques are avoided by relying on temperature measurements to change the path, without changing the path length, of laser beams within a ring laser gyroscope in a predetermined and calculated manner.